This invention relates deals with fiberboard containers, such as those made from corrugated board, which are of the larger sizes intended mainly for packing and shipping bulk material and which are usually carried on pallets. Many of these containers are of square or rectangular from and the vertical wall panels thereof tend to bulge transversely and in a vertical direction when they are stacked. This is due to the fact that the compressive strength of the box is derived mainly from the vertical fold corners thereof. Attempts have been made in the prior art to increase the compressive strength by changing the cross-sectional shape of the container from quadrangular to a polygonal shape having more than four corner fold lines or joints so as to increase its compressive strength and thereby resistance to bulging upon stacking. However, usually this causes difficulty in setting-up the tubular body of the container from the flat knocked-down condition in which it is supplied, as it is a struggle to get it to assume and retain the proper polygonal shape unitl it is fastened in that shape. In many cases, it is necessary to provide a separate end tray of proper polygonal form in which the tubular body is set and secured to obtain and retain its shape.